Shaping light with GPUs Damien Gratadour Observatoire de Paris & Université Paris Diderot 1
Observing stars from the ground ๏ Atmospheric turbulence ๏ Disturbs the trajectory of light rays when they cross the atmosphere ๏ Reduces astronomical images quality ๏ Similar to the effect of aberrations in an optical system ๏ Adaptive optics ๏ Compensate in real-time for the effect of optical aberrations on image quality ๏ Already in use on most 5-10m astronomical telescope to provide nominal image quality whatever the turbulence conditions 2
Observing stars from the ground ๏ From a spherical wave to a flat wavefront 3
Observing stars from the ground ๏ Crossing the atmosphere distorts the wavefront ๏ Mixture of hot and cold air bubbles with varying refractive index ๏ Strongly affects image quality 4
Adaptive optics Disturbed wavefront ๏ Compensate in real-time Deformable mirror the wavefront perturbations Real-time controller ๏ Using a wavefront Beam- splitter sensor to measure them Corrected wavefront ๏ Using a deformable Wavefront mirror to reshape sensor High resolution the wavefront camera ๏ Commands to the mirror must be computed in real-time (1ms rate) 5 Loop closed Loop open
Adaptive optics ๏ Example with observations of the moon using a 8m telescope Without AO With AO 6
European Extremely Large Telescope ๏ 39m diameter telescope : x5 in diameter => x25 in system complexity ๏ 100m dome, 2800 tones structure rotating @ 360°, seismic safe (Chile) ๏ 1.2 G€ project, first light foreseen in 2024 ๏ Construction led by ESO (European Southern Observatory), international organisation funded by 15 European countries ๏ Telescope components + science instruments built by european research labs + industrial partners 7
Adaptive optics Disturbed wavefront ๏ Compensate in real-time Deformable mirror the wavefront perturbations Real-time controller ๏ Using a wavefront Beam- splitter sensor to measure them Corrected wavefront ๏ Using a deformable Wavefront mirror to reshape sensor High resolution the wavefront camera ๏ Commands to the mirror must be computed in real-time (1ms rate) 8 Loop closed Loop open
AO real-time controller ๏ Highly heterogeneous HPC facility 9
The Green Flash project ๏ Goal : prototype a generic RTC for the next generation of AO on extremely large telescopes ๏ 4 partners in Europe (2 academic partners + 2 SMEs), project lead : Observatoire de Paris, 3.8 M€ investment funded under the H2020 program (FET-HPC, project #671662) ๏ Assess various technologies (CPUs, GPUs, FPGAs) for the different sub-components (real-time core, supervisor) and find the best trade-off ๏ Assemble a full featured prototype in the lab by 2018 ๏ 10
Prototyping in the lab ๏ AO system components for the E-ELT are not yet available ๏ High framerate low noise cameras under development ๏ High density deformable mirror under construction, too large to be integrated in the lab (2.5m) Credits : Microgate ๏ Need to emulate these components to work in the lab 11
Prototyping in the lab ๏ Liquid Crystal On Silicon : reflective Spatial Light Modulator ๏ Changes the phase of light without any change in intensity according to alignment of LC ๏ LC controlled pixel by pixel by applied voltage using CMOS backplane ๏ Controlled through DVI signal from a PC ๏ Large number of pixels (up to 1k x 1k) ๏ 120 FPS (DVI), rise time 5ms Credits : Hamamatsu 12
First stage lab experiment ๏ Integration of a pyramid wavefront sensor demonstrator Credits : S. Egner 13
First stage lab experiment ๏ Integration of a pyramid wavefront sensor demonstrator ๏ Using a 10 GbE camera from Emergent Vision Technologies (emulate a single WFS at full frame rate) ๏ Zoom optics : allows for various pupil samplings, i.e. various system dimensioning 14
Loop concept ๏ Emulate a simple AO loop RTC Map wavefront to given DM geometry Get wavefront measurements Reconstruct the phase High framerate, low latency data acquisition 15
First stage lab experiment ๏ Custom RTC demonstrator ๏ High end dual GPU server 16
First stage lab experiment ๏ Critical aspect : low latency data acquisition from the camera ๏ Using an off-the-shelf frame grabber 10 Gbe Frame-grabber DDR DDR Mem. Mem. Pixel data FPGA Serial GPU DMA interface engine PCIe bus DDR CPU Mem. ๏ Very high jitter in performance 17
First stage lab experiment ๏ Using GPU direct + persistent kernels : reduce jitter, i.e. ensure loop stability 10 Gbe Frame-grabber DDR DDR Cpy Cpy Cpy Cpy Cpy Mem. Mem. Comp Comp Comp Comp Comp Pixel data Timeline for standard kernel call FPGA Serial GPU DMA interface engine PCIe bus Cpy Cpy Cpy Cpy Cpy Comp Comp Comp Comp Comp DDR Timeline for persistent kernel call CPU Mem. 18
First stage lab experiment ๏ Custom frame grabber : developped in collaboration with PLDA ๏ Based on Altera Stratix V board ๏ Using PLDA development tools for PCIe + UDP ๏ Multiple DMA engines QuickUDP Buffer GVCP DMA0 CPU DEMUX PHY UDP0 Buffer GVSP DMA1 GPU Buffer GVCP FIBRE DMA2 Signal TAP CPU Logic Registers Analyzer Address App config DEC Translation . . Buffer DMA3 FIFO PHY CPU UDP1 . QuickPCIe QuickUDP Data Generator Custom 10 Gbe GigeVision Framegrabber 19
What's next ? ๏ Close the loop and demonstrate AO performance for various mirror scales / actuators geometry ๏ Build a scaled down prototype of real system with several cameras (up to 6) : need to increase the number of GPUs in the system (15 to 20 boards) ๏ Compare performance of GPU solution against other technologies (CPUs, FPGAs) at full scale ๏ Have more fun ! 20
Outline ๏ Next generation of large ground based telescope will require AO systems with unprecedented complexity (x25 as compared to state of the art) ๏ Several generations of instruments => several system dimensioning ๏ GPUs are good candidates to build the real-time controllers for these systems ๏ Single board performance + scalability ๏ Requires full control over pixels data acquisition to get deterministic performance (i.e. loop stability) and dedicated data pipeline ๏ GPUs are also used to simulate systems performance and lead trade-off studies for the design (see H. Ltaief talk, room 212A, 3 p.m. today) ๏ 21
Thank you 22
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